Bug: folder-tree-dispatcher-thread-affinity

[drmoisan]

• Work Mode: full-bug

Summary

The shared Outlook folder-tree service can invoke WpfDispatcherYield from a dispatcher-free worker after EmailDataMiner starts a cold cache build through Task.Run. Dispatcher.Yield then raises InvalidOperationException, and a fallback that only uses Task.Yield would not restore the required Outlook STA traversal contract.

Environment

• OS/version: Windows with Outlook VSTO runtime
• Application: TaskMaster Outlook add-in
• Trigger: Cold folder-tree cache build or refresh that reaches a deadline-controlled yield point
• Data source or fixture: Outlook folder hierarchy through the live COM adapter

Steps to Reproduce

1. Start the email-mining path while the shared folder-tree cache requires a build or refresh.
2. EmailDataMiner.MineEmails invokes ExtractOlFolderChunks through Task.Run.
3. The folder-tree reader reaches WpfDispatcherYield.YieldAsync after the deadline clock requests a yield.

Expected Behavior

Live Outlook COM traversal and cooperative yielding remain on the captured Outlook STA dispatcher, and the caller receives a folder snapshot without a thread-affinity exception.

Actual Behavior

WpfDispatcherYield invokes Dispatcher.Yield(DispatcherPriority.Background) on a worker thread that has no current dispatcher. WPF raises InvalidOperationException indicating that the calling thread does not have a current Dispatcher.

Logs / Screenshots

• Captured stack evidence reviewed
• Snippet: EmailDataMiner -> OutlookFolderTreeService -> FolderTreeSnapshotBuilder -> OutlookFolderHierarchyReader -> WpfDispatcherYield -> Dispatcher.Yield

Impact / Severity

• Blocker
• High
• Medium
• Low

Source

From: docs/features/potential/2026-08-04-folder-tree-dispatcher-thread-affinity.md

[drmoisan]

Delivered by #422 (merged 2026-08-07, merge commit 0d85748) and the follow-up #423 (merged 2026-08-07, merge commit fb32b92). Neither PR body used a closing keyword, so this issue was not auto-closed.

Verified against origin/main:

• UtilitiesCS/OutlookObjects/Folder/WpfDispatcherYield.cs no longer calls Dispatcher.Yield(). It resolves a dispatcher explicitly as Dispatcher.FromThread(Thread.CurrentThread) ?? UtilitiesCS.UiThread.Dispatcher, posts a background-priority InvokeAsync, and throws InvalidOperationException with an actionable message when no dispatcher is available.
• Folder-tree service composition, cold builds, refreshes, and notification cleanup are marshaled through the captured Outlook STA dispatcher, so the thread-first resolution preserves the STA traversal contract rather than falling back to the process-global dispatcher.
• Regression coverage exists at WpfDispatcherYieldTests.YieldAsync_WithoutDispatcher_RemainsStrict and OutlookFolderTreeServiceConcurrencyTests.GetSnapshotAsync_WorkerOriginatedColdBuild_UsesCapturedStaDispatcher.

Expected behavior in this issue (traversal and yielding remain on the captured Outlook STA dispatcher, no thread-affinity exception) is satisfied.

Closing as delivered.